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Synthesis of nickel aluminides under high-pressure shock loading
Volume 3, number 9,10
MATERIALS LE’lTERS
July 1985
SYNTHESIS OF NICKEL ALUMINIDES UNDER HIGH-PRESSURE SHOCK LOADING Y. HORIE a, R.A. GRAHAM b and I.K. SIMONSEN ’
a Department
of Civil Engineering, North Carolina State University, Raleigh, NC 27650, USA b Sandia National Laboratories, Albuquerque, NM 87185, USA ’ Division of Engineering Research, North Carolina State University, Raleigh, NC 27650, USA
Received 17 April 1985
We report the first shock synthesis of nickel aluminides by explosive loading of the blended mixture of elemental nickel and aluminum powders. Insofar as we are aware, this is the first synthesis of these compounds from fully dense powder compacts. The major end product was ordered Ni,Al which was identified by X-ray diffraction and electron-probe microanalysis. Transitional reaction zones or heat-affected zones contained NiAl, Ni,Al,, and NiAI,. Microhardness (DPH) of Ni,Al was 438 f 8. This is the same hardness obtained for cold-rolled or rapidly solidified Ni, Al containing boron additions.
Intermetallic compounds such as Ni3Al have highly desirable characteristics of high-temperature strength and corrosion and oxidation resistance [ 1,2]. Recent discoveries which improve their ductility by boron additives have renewed the interest in their development as high-temperature structural materials [3]. Historically, a variety of processing techniques has been used for producing intermetallic compounds. It could be cast from the melt, or synthesized through a self-propagating, high-temperature method [4-6], or formed by a diffusion process during pack cementation [2,7]. The high-pressure shock-synthesis process places material in a highly unusual combination of states. Shock synthesis of diamond powders from graphite provides the preeminent industrial example. The combination of material states achieved in shock synthesis is not achieved by any other synthesis process even though certain individual aspects of shock compression may be subject to duplication, for example, with static high pressure. Not only are very high pressure and significant increases in temperature achieved on the microsecond timescale, but the process involves large local stress and temperature gradients, forced relative mass motion, mechanically induced saturation levels of point and line defects, exposure of fresh surfaces, and cleaning of existing surfaces. The high-pressure pulse produces a fully dense compact from an initially porous compact. Greatly enhanced solid-state
354
reactivity may be encountered under these circumstances and leads to the possibility of synthesizing known substances with unusual properties, known metastable substance, or new substances not known in prior synthesis studies. The technological potential of such an unusual process has been widely recognized
PI. This report describes the first observation of the shock synthesis of nickel aluminides. Although the resulting alloys are known from prior work, the present synthesis is thought to be the first in a fully dense powder mixture of blended powders. The specimen mixtures, which were blended mechanically, had a nominal composition of 30 ~01% Al consisting of CERAC A-l 189/-325 mesh aluminum powder and CERAC N-1095/-200+325 mesh nickel powder. Average particle size of these powders provided by CERAC is 5-15 /J-II and 44-74 W, respectively. The powder compacts were pressed in place in the recovery furtures to a density of 4.25 Mg/m3, about 60% of solid density. The powders were not purified before the pressing. The shock-synthesis experiments were carried out in the Sandia recovery fatures [9] with conditions as shown in table 1. The futures utilize explosive plane-wave loading into particular specimen recovery capsules which have characteristic responses to the loading. Each recovery capsule is carefully studied 0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 3, number 9,lO
MATERIALS LETTERS
July 1985
Table 1 Shock conditions in powder compacts
___~_
Experiment
Fixture
Peak pressure range mean GPa)
Duration (@I
_______ Peak temperature (“9
71G846 726846
Momma Bear A Momma Bear A
14-16 19-22
4.5 4.7
450 590
with realistic numerical simulation which defines the shock compression conditions within the powder compacts. The compression process is found to be controlled by the large porosity (40%) of the compacts. Based on extensive numerical simulations, agreement between observations in various fixtures, studies at various specimen compact densities, and visual inspection of shocked compacts in the study of 25 powder materials, it is felt that the pressures are predicted to an accuracy and reproducibility of ~10% while temperatures are accurate to =20%. A high-temperature region is found to be present in the peripheral region due to pressure pulses moving quickly around the outer edge of the powder compacts. The presence of this hot region is readily discernible in most compacts and plays a critical role in the present synthesis. The capsules were recovered after the explosive loading and the outer copper parts were sawed off, revealing a strong compact with strong bonding of the compact to the top and bottom copper plugs. Radial sections were cut from the compacts and examined with electron-probe microanalysis (EPMA). For large areas of ordered NijAl, we were able to use X-ray diffraction (Cu Kcrt) to identify the compound by using sample material filed off from the corresponding surface areas. Figs. 1 and 2 show the typical regions on the sectional compacts in which nickel aluminides were observed. These sections were mounted in lucite and metallographically prepared with a final polish abrasive of 0.05 pm A1203. They were unetched, and the micrographs were taken with a Reichert metallograph. The microhardness measurements were also made on the Reichert. The formation of the intermetallic compounds in experiment 716846 was only observed in a small triangular region close to the periphery of the specimen
.____
._~
Fig. 1. Formation of nickel aluminides in experiment 71G846. Reactions are seen in the upper right hand corner. The length of the bar is 1 mm.
disk. This is consistent with the prediction from numerical simulation that higher temperatures are located in the outer region of the sample. Apparently, the shock pressure and temperature conditions of 7 1G846 are close to a threshold condition for initiating reactions in the shock-activated Ni-Al system. Reactions in experiment 72G846, which was loaded by a somewhat stronger shock (see table l), were stronger but limited to three regions within the compact. They were, as shown in figs. 2 and 3 : (A) A triangular region similar to that found in 7 1G846, but with a much larger area extending almost entire width of the periphery. (B) A very thin region, having a width of x0.5 mm along the center line of the disk. (C) A horizontal region between the base plug and the space separated by a spalled crack. The bulk of the compact in the central region was unreacted. 35.5
Volume 3, number 9,lO
MATERIALS LETTERS
July 1985
Fig. 2. Formation of nickel aluminides in experiment 72G846. Reaction zones are illustrated in fig. 3. The length of the bar is 1 mm.
The bulk of region A consists of an almost homogeneous block of Ni,Al and residual, irregularly shaped Ni “particles”. There were no discernible solid solutions in the spaces between Ni,Al and the unreacted Ni particles. We believe that the residual nickel is a result of the inhomogeneity in the initial distribution of Al and Ni particles. The end product Ni3Al is believed to be a result of the initial mixture which is close to the stoichiometry of Ni,Al. Other compounds in the Ni-Al system were found in transition regions between Ni,Al in region A and unreacted regions in the center of the sample. In all transition regions one can further discern two subregions: the outer region consisting predominately of Ni2A13 surrounding eutectic phase of NiAl, and Al, and the inner region (next to Ni3Al) of NiAl and Ni3Al grains. Ni2A13 appears as a thin layer on the surface of nickel particles. In contrast, NiA13 grains appear in the originally aluminum matrix with a typical eutectic structure and grow over the Ni2A13 layer. It is likely that the NiAl3 grains are formed by precipitation from a liquid state caused by heating from exothermic reactions in adjacent areas. The compounds NiAl and NiTAl exist only in small quantities and have not been investigated by microdiffraction analysis. Therefore, although the growth pattern of Ni2A13 is consistent with the findings of other investigations on the Ni-Al system [4,7, lo- 121, one must consider the possibility that Ni2A13 is Al-rich NiAl. They are known to have the same structure, derived from bee, with only a differ356
ent distribution of the nickel vacancies [7]. The latter interpretation is not inconsistent with the microhardness data on the present materials in table 2 which may be compared with Westbrook’s results on NM. The difference in hardness was related to the type of defects in Ni-Al [l] : vacancies dominate high aluminum composition while substitutional defects dominate high nickel compositions. It is also noted that the hardness of Ni,Al is of the magnitude obtained for cold-rolled or rapidly solidified NigAl containing boron additions [3]. The initiation of the reactions in the hotter region of the compacts shows the strong influence of temperature. Each of the reacted regions except region C corresponds to a region of high temperature predicted by numerical simulation shown in fig. 3. Figs. 2 and 3 show that a nominal temperature of MOO’C is required for initiation. This temperature is about the same as the value (550°C) reported for the initiation of an intensive exothermic reaction in low-density mixtures of annealed Ni and Al powders in the selfpropagating synthesis (SHS) process [4]. It is also significant that the very thin region of NiAl (or Ni3A12) Table 2 Microhardness data a) on shock-synthesized
nickel aluminides
Ni
NisAl
NiAl
179 * 5.6
438 f 8
621+10
a) DPH, 30 pd load.
_ ,..
/
,-
----
IN “C
SOME Ni-AI-CU
---m-rrll
ALL
+ At + SO~IMENi2AI3
-
(.<.
_
-.._:.-
_
Fig.3. G&ulatedtemperature distribution in combination with zones of synthesis in experiment 72G846.
TEMPERATURES
NiAIs
_-
-.s_~.-
Volume 3, number 9,lO
MATERIALS LETTERS
along the center axis corresponds to temperatures above 500°C predicted by the numerical simulation. The initiation of the reaction in region C cannot be correlated with the high-temperature condition in region A. However, as evidenced by a deep penetration of Cu in region C (up to the spalled crack), Cu appears to act as an intermediary in the ignition of the Ni-Al system. This relates well with an experimental observation [ 141 that combustion in the Ni-Al system begins at ~350°C and after ignition the reaction temperature jumps to ~950°C. The copper ignition mechanism is also corroborated by the calculated temperature estimate shown in fig. 3. In agreement with the Cu-Al combustion, the corresponding maximum temperature in 716846 is 250°C and no reactions have been observed in region C in this experiment. In unreacted regions there were sporadic, thin-alloyed veins which were smaller than the spatial resolution of our EPMA equipment (3-5 m). They have a color and texture of Ni2A13. Apparently, there was not enough energy deposition to sustain further reactions. The present observations are among the first quantitative shock chemistry studies, and there is not yet sufficient understanding to specify the chemical reaction process. But other related processes such as SHS or intense pulsed electron beam heating [lo] provide some guidance as to the uniqueness of the shock process. The sequence of events thought to be important in chemical synthesis with blended powder mixtures under shock compression are: (a) Initial pressure pulse reverberations within individual particles with visco-plastic deformation, highspeed motion of dislocations, formation of defects, cleaning of surfaces and possibly opening of fresh surfaces. (b) Consolidation of particles into a fully dense state due to plastic deformation and possibly spalling of particles. At this stage the particles are mechanically “activated” and in intimate contact. Although a “pressure” and overall temperature is characteristic of macroscopic regions, local stress and temperature gradients may be severe, and atomic and microscopic level relative mass motion may be forced by large acceleration forces. (c) Solid-state diffusion of Al and/or Ni or dissolution of Ni in liquid aluminum may proceed at enhanced 358
July 1985
rates due to the mechanical activation. (d) Local chemical reaction zones form and lead to exothermic reactions and correspondingly large increases in temperature. (e) Formation of reaction products appropriate for pressure, temperature and defect state of the materials. (f) Crystallization of end products. The observed location of reaction products within the compacts and their correlation with areas of high shock-induced temperature clearly shows that initiation of the reactions occurred during the few-microsecond period while the specimens were under loading. But the time frame in which the reactions go to completion is not so certain. The reactions are expected to be exothermic, and the resulting increase in temperature of 1000 to 15OOO’C might be expected to propagate throughout the specimen in the manner observed in SHS. The location of the products clearly indicates that the reaction did not propagate through substantial distances; either the reaction was quenched (due to the release of pressure) or other factors (such as the fully dense state) prevented propagation of reaction. SHS experiments on nickel aluminides have shown that reactions will not propagate if porosity is less than ~20% [4,6]. The microhardness of Ni,Al noted earlier provides another clue to the uniqueness of the shock process. Since the temperature of the reactions is fairly high, cold working is not likely to be the mechanism for this hardness. Therefore, if the hardening of the shocksynthesized N$Al were related to the rate of cooling similar to those for rapidly solidified materials, it should be recognized that the only quenching process is associated with rapid unloading stress waves. The pores that surround region A may be related to the interaction with such unloading waves. Similar experiments have been conducted with powder mixtures of titanium and aluminum, but the synthesized products are of limited quantities. The X-ray diffraction study shows that the products may be new phases not previously observed in other synthesis processes. The present results show that chemical synthesis of intermetallic compounds can be carried out under shock compression conditions. The unusual combination of material states achieved in the process holds promise for synthesis of bulk intermetallic compounds with unusual properties and the synthesis of new compounds.
Volume 3, number 9,lO
MATERIALS LETTERS
The work at Sandia was partially supported by the U.S. Department of Energy under the contract number DE-AC04-76DP00789. The work at NCSU was partially supported by the NCSU and the LLNL under subcontract number 5719205. We are pleased to acknowledge the technical assistance of K. Elsner, C.J. Daniel, and Professor H.H. Stadelmeier, and the review by Professor C.C. Koch.
References [ 1] J.H. Westbrook, Mechanical properties of intermetallic compounds (Wiley, New York, 1959). [ 21 D. Chatterji, R.C. DeVries and G. Romeo, Advan. Corros. Sci. Technol. 6 (1976) 1. [3] C.T. Liu and J.O. Stiegler, Science 226 (1984) 636. [4] Yu.S. Naiborodenko and V.I. Itin, Combust. Explos. Shock Waves 11 (1976) 293,626. [5] A.P. Hardt and P.V. Phung, Combust. Flame 21(1973) 77.
July 1985
[6] P.V. Phung and A.P. Hardt, Combust. Flame 22 (1974) 323. [7] A. Thevand, S. Poize, J.P. Crousier and R. Streiff, J. Mat. Sci. 16 (1981) 2467. [8] B. Morosin and R.A. Graham, in: Shock waves in condense media - 1981, eds. W.J. Nellis, L. Seaman and R.A. Graham (AIP, New York, 1982). [9] R.A. Graham and D.M. Webb, in: Shock waves in condensed matter - 1983, eds. J.R. Asay, R.A. Graham and G.L. Straub (North-Holland, Amsterdam, 1984) pp. 211-214. [lo] M.A. Korchagin, V.V. Aleksandrov and J.A. Neromov, Izv. Sibir. Otd. Akad. Nauk SSSR (Khim.) 6 (1979) 104. [ 1 I] M.M. Janssen and G.D. Rieck, Trans. Met. Sot. AIME 239 (1967) 1371. [12] M.M. Janssen, Met. Trans. 4 (1973) 1623. [ 131 CC. Koch, private communication. [ 141 V.I. Itin, A.D. Bratchikov and A.V. Lepinskikh, Combust. Explos. Shock Waves 17 (1981) 506. [ 151 R.F. Davis, Y. Horie, R.O. Scattergood and H. Palmour III, in: Advances in ceramics, ed. W.D. Kingery (Plenum Press, New York), to be published.
3.59
MATERIALS LE’lTERS
July 1985
SYNTHESIS OF NICKEL ALUMINIDES UNDER HIGH-PRESSURE SHOCK LOADING Y. HORIE a, R.A. GRAHAM b and I.K. SIMONSEN ’
a Department
of Civil Engineering, North Carolina State University, Raleigh, NC 27650, USA b Sandia National Laboratories, Albuquerque, NM 87185, USA ’ Division of Engineering Research, North Carolina State University, Raleigh, NC 27650, USA
Received 17 April 1985
We report the first shock synthesis of nickel aluminides by explosive loading of the blended mixture of elemental nickel and aluminum powders. Insofar as we are aware, this is the first synthesis of these compounds from fully dense powder compacts. The major end product was ordered Ni,Al which was identified by X-ray diffraction and electron-probe microanalysis. Transitional reaction zones or heat-affected zones contained NiAl, Ni,Al,, and NiAI,. Microhardness (DPH) of Ni,Al was 438 f 8. This is the same hardness obtained for cold-rolled or rapidly solidified Ni, Al containing boron additions.
Intermetallic compounds such as Ni3Al have highly desirable characteristics of high-temperature strength and corrosion and oxidation resistance [ 1,2]. Recent discoveries which improve their ductility by boron additives have renewed the interest in their development as high-temperature structural materials [3]. Historically, a variety of processing techniques has been used for producing intermetallic compounds. It could be cast from the melt, or synthesized through a self-propagating, high-temperature method [4-6], or formed by a diffusion process during pack cementation [2,7]. The high-pressure shock-synthesis process places material in a highly unusual combination of states. Shock synthesis of diamond powders from graphite provides the preeminent industrial example. The combination of material states achieved in shock synthesis is not achieved by any other synthesis process even though certain individual aspects of shock compression may be subject to duplication, for example, with static high pressure. Not only are very high pressure and significant increases in temperature achieved on the microsecond timescale, but the process involves large local stress and temperature gradients, forced relative mass motion, mechanically induced saturation levels of point and line defects, exposure of fresh surfaces, and cleaning of existing surfaces. The high-pressure pulse produces a fully dense compact from an initially porous compact. Greatly enhanced solid-state
354
reactivity may be encountered under these circumstances and leads to the possibility of synthesizing known substances with unusual properties, known metastable substance, or new substances not known in prior synthesis studies. The technological potential of such an unusual process has been widely recognized
PI. This report describes the first observation of the shock synthesis of nickel aluminides. Although the resulting alloys are known from prior work, the present synthesis is thought to be the first in a fully dense powder mixture of blended powders. The specimen mixtures, which were blended mechanically, had a nominal composition of 30 ~01% Al consisting of CERAC A-l 189/-325 mesh aluminum powder and CERAC N-1095/-200+325 mesh nickel powder. Average particle size of these powders provided by CERAC is 5-15 /J-II and 44-74 W, respectively. The powder compacts were pressed in place in the recovery furtures to a density of 4.25 Mg/m3, about 60% of solid density. The powders were not purified before the pressing. The shock-synthesis experiments were carried out in the Sandia recovery fatures [9] with conditions as shown in table 1. The futures utilize explosive plane-wave loading into particular specimen recovery capsules which have characteristic responses to the loading. Each recovery capsule is carefully studied 0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 3, number 9,lO
MATERIALS LETTERS
July 1985
Table 1 Shock conditions in powder compacts
___~_
Experiment
Fixture
Peak pressure range mean GPa)
Duration (@I
_______ Peak temperature (“9
71G846 726846
Momma Bear A Momma Bear A
14-16 19-22
4.5 4.7
450 590
with realistic numerical simulation which defines the shock compression conditions within the powder compacts. The compression process is found to be controlled by the large porosity (40%) of the compacts. Based on extensive numerical simulations, agreement between observations in various fixtures, studies at various specimen compact densities, and visual inspection of shocked compacts in the study of 25 powder materials, it is felt that the pressures are predicted to an accuracy and reproducibility of ~10% while temperatures are accurate to =20%. A high-temperature region is found to be present in the peripheral region due to pressure pulses moving quickly around the outer edge of the powder compacts. The presence of this hot region is readily discernible in most compacts and plays a critical role in the present synthesis. The capsules were recovered after the explosive loading and the outer copper parts were sawed off, revealing a strong compact with strong bonding of the compact to the top and bottom copper plugs. Radial sections were cut from the compacts and examined with electron-probe microanalysis (EPMA). For large areas of ordered NijAl, we were able to use X-ray diffraction (Cu Kcrt) to identify the compound by using sample material filed off from the corresponding surface areas. Figs. 1 and 2 show the typical regions on the sectional compacts in which nickel aluminides were observed. These sections were mounted in lucite and metallographically prepared with a final polish abrasive of 0.05 pm A1203. They were unetched, and the micrographs were taken with a Reichert metallograph. The microhardness measurements were also made on the Reichert. The formation of the intermetallic compounds in experiment 716846 was only observed in a small triangular region close to the periphery of the specimen
.____
._~
Fig. 1. Formation of nickel aluminides in experiment 71G846. Reactions are seen in the upper right hand corner. The length of the bar is 1 mm.
disk. This is consistent with the prediction from numerical simulation that higher temperatures are located in the outer region of the sample. Apparently, the shock pressure and temperature conditions of 7 1G846 are close to a threshold condition for initiating reactions in the shock-activated Ni-Al system. Reactions in experiment 72G846, which was loaded by a somewhat stronger shock (see table l), were stronger but limited to three regions within the compact. They were, as shown in figs. 2 and 3 : (A) A triangular region similar to that found in 7 1G846, but with a much larger area extending almost entire width of the periphery. (B) A very thin region, having a width of x0.5 mm along the center line of the disk. (C) A horizontal region between the base plug and the space separated by a spalled crack. The bulk of the compact in the central region was unreacted. 35.5
Volume 3, number 9,lO
MATERIALS LETTERS
July 1985
Fig. 2. Formation of nickel aluminides in experiment 72G846. Reaction zones are illustrated in fig. 3. The length of the bar is 1 mm.
The bulk of region A consists of an almost homogeneous block of Ni,Al and residual, irregularly shaped Ni “particles”. There were no discernible solid solutions in the spaces between Ni,Al and the unreacted Ni particles. We believe that the residual nickel is a result of the inhomogeneity in the initial distribution of Al and Ni particles. The end product Ni3Al is believed to be a result of the initial mixture which is close to the stoichiometry of Ni,Al. Other compounds in the Ni-Al system were found in transition regions between Ni,Al in region A and unreacted regions in the center of the sample. In all transition regions one can further discern two subregions: the outer region consisting predominately of Ni2A13 surrounding eutectic phase of NiAl, and Al, and the inner region (next to Ni3Al) of NiAl and Ni3Al grains. Ni2A13 appears as a thin layer on the surface of nickel particles. In contrast, NiA13 grains appear in the originally aluminum matrix with a typical eutectic structure and grow over the Ni2A13 layer. It is likely that the NiAl3 grains are formed by precipitation from a liquid state caused by heating from exothermic reactions in adjacent areas. The compounds NiAl and NiTAl exist only in small quantities and have not been investigated by microdiffraction analysis. Therefore, although the growth pattern of Ni2A13 is consistent with the findings of other investigations on the Ni-Al system [4,7, lo- 121, one must consider the possibility that Ni2A13 is Al-rich NiAl. They are known to have the same structure, derived from bee, with only a differ356
ent distribution of the nickel vacancies [7]. The latter interpretation is not inconsistent with the microhardness data on the present materials in table 2 which may be compared with Westbrook’s results on NM. The difference in hardness was related to the type of defects in Ni-Al [l] : vacancies dominate high aluminum composition while substitutional defects dominate high nickel compositions. It is also noted that the hardness of Ni,Al is of the magnitude obtained for cold-rolled or rapidly solidified NigAl containing boron additions [3]. The initiation of the reactions in the hotter region of the compacts shows the strong influence of temperature. Each of the reacted regions except region C corresponds to a region of high temperature predicted by numerical simulation shown in fig. 3. Figs. 2 and 3 show that a nominal temperature of MOO’C is required for initiation. This temperature is about the same as the value (550°C) reported for the initiation of an intensive exothermic reaction in low-density mixtures of annealed Ni and Al powders in the selfpropagating synthesis (SHS) process [4]. It is also significant that the very thin region of NiAl (or Ni3A12) Table 2 Microhardness data a) on shock-synthesized
nickel aluminides
Ni
NisAl
NiAl
179 * 5.6
438 f 8
621+10
a) DPH, 30 pd load.
_ ,..
/
,-
----
IN “C
SOME Ni-AI-CU
---m-rrll
ALL
+ At + SO~IMENi2AI3
-
(.<.
_
-.._:.-
_
Fig.3. G&ulatedtemperature distribution in combination with zones of synthesis in experiment 72G846.
TEMPERATURES
NiAIs
_-
-.s_~.-
Volume 3, number 9,lO
MATERIALS LETTERS
along the center axis corresponds to temperatures above 500°C predicted by the numerical simulation. The initiation of the reaction in region C cannot be correlated with the high-temperature condition in region A. However, as evidenced by a deep penetration of Cu in region C (up to the spalled crack), Cu appears to act as an intermediary in the ignition of the Ni-Al system. This relates well with an experimental observation [ 141 that combustion in the Ni-Al system begins at ~350°C and after ignition the reaction temperature jumps to ~950°C. The copper ignition mechanism is also corroborated by the calculated temperature estimate shown in fig. 3. In agreement with the Cu-Al combustion, the corresponding maximum temperature in 716846 is 250°C and no reactions have been observed in region C in this experiment. In unreacted regions there were sporadic, thin-alloyed veins which were smaller than the spatial resolution of our EPMA equipment (3-5 m). They have a color and texture of Ni2A13. Apparently, there was not enough energy deposition to sustain further reactions. The present observations are among the first quantitative shock chemistry studies, and there is not yet sufficient understanding to specify the chemical reaction process. But other related processes such as SHS or intense pulsed electron beam heating [lo] provide some guidance as to the uniqueness of the shock process. The sequence of events thought to be important in chemical synthesis with blended powder mixtures under shock compression are: (a) Initial pressure pulse reverberations within individual particles with visco-plastic deformation, highspeed motion of dislocations, formation of defects, cleaning of surfaces and possibly opening of fresh surfaces. (b) Consolidation of particles into a fully dense state due to plastic deformation and possibly spalling of particles. At this stage the particles are mechanically “activated” and in intimate contact. Although a “pressure” and overall temperature is characteristic of macroscopic regions, local stress and temperature gradients may be severe, and atomic and microscopic level relative mass motion may be forced by large acceleration forces. (c) Solid-state diffusion of Al and/or Ni or dissolution of Ni in liquid aluminum may proceed at enhanced 358
July 1985
rates due to the mechanical activation. (d) Local chemical reaction zones form and lead to exothermic reactions and correspondingly large increases in temperature. (e) Formation of reaction products appropriate for pressure, temperature and defect state of the materials. (f) Crystallization of end products. The observed location of reaction products within the compacts and their correlation with areas of high shock-induced temperature clearly shows that initiation of the reactions occurred during the few-microsecond period while the specimens were under loading. But the time frame in which the reactions go to completion is not so certain. The reactions are expected to be exothermic, and the resulting increase in temperature of 1000 to 15OOO’C might be expected to propagate throughout the specimen in the manner observed in SHS. The location of the products clearly indicates that the reaction did not propagate through substantial distances; either the reaction was quenched (due to the release of pressure) or other factors (such as the fully dense state) prevented propagation of reaction. SHS experiments on nickel aluminides have shown that reactions will not propagate if porosity is less than ~20% [4,6]. The microhardness of Ni,Al noted earlier provides another clue to the uniqueness of the shock process. Since the temperature of the reactions is fairly high, cold working is not likely to be the mechanism for this hardness. Therefore, if the hardening of the shocksynthesized N$Al were related to the rate of cooling similar to those for rapidly solidified materials, it should be recognized that the only quenching process is associated with rapid unloading stress waves. The pores that surround region A may be related to the interaction with such unloading waves. Similar experiments have been conducted with powder mixtures of titanium and aluminum, but the synthesized products are of limited quantities. The X-ray diffraction study shows that the products may be new phases not previously observed in other synthesis processes. The present results show that chemical synthesis of intermetallic compounds can be carried out under shock compression conditions. The unusual combination of material states achieved in the process holds promise for synthesis of bulk intermetallic compounds with unusual properties and the synthesis of new compounds.
Volume 3, number 9,lO
MATERIALS LETTERS
The work at Sandia was partially supported by the U.S. Department of Energy under the contract number DE-AC04-76DP00789. The work at NCSU was partially supported by the NCSU and the LLNL under subcontract number 5719205. We are pleased to acknowledge the technical assistance of K. Elsner, C.J. Daniel, and Professor H.H. Stadelmeier, and the review by Professor C.C. Koch.
References [ 1] J.H. Westbrook, Mechanical properties of intermetallic compounds (Wiley, New York, 1959). [ 21 D. Chatterji, R.C. DeVries and G. Romeo, Advan. Corros. Sci. Technol. 6 (1976) 1. [3] C.T. Liu and J.O. Stiegler, Science 226 (1984) 636. [4] Yu.S. Naiborodenko and V.I. Itin, Combust. Explos. Shock Waves 11 (1976) 293,626. [5] A.P. Hardt and P.V. Phung, Combust. Flame 21(1973) 77.
July 1985
[6] P.V. Phung and A.P. Hardt, Combust. Flame 22 (1974) 323. [7] A. Thevand, S. Poize, J.P. Crousier and R. Streiff, J. Mat. Sci. 16 (1981) 2467. [8] B. Morosin and R.A. Graham, in: Shock waves in condense media - 1981, eds. W.J. Nellis, L. Seaman and R.A. Graham (AIP, New York, 1982). [9] R.A. Graham and D.M. Webb, in: Shock waves in condensed matter - 1983, eds. J.R. Asay, R.A. Graham and G.L. Straub (North-Holland, Amsterdam, 1984) pp. 211-214. [lo] M.A. Korchagin, V.V. Aleksandrov and J.A. Neromov, Izv. Sibir. Otd. Akad. Nauk SSSR (Khim.) 6 (1979) 104. [ 1 I] M.M. Janssen and G.D. Rieck, Trans. Met. Sot. AIME 239 (1967) 1371. [12] M.M. Janssen, Met. Trans. 4 (1973) 1623. [ 131 CC. Koch, private communication. [ 141 V.I. Itin, A.D. Bratchikov and A.V. Lepinskikh, Combust. Explos. Shock Waves 17 (1981) 506. [ 151 R.F. Davis, Y. Horie, R.O. Scattergood and H. Palmour III, in: Advances in ceramics, ed. W.D. Kingery (Plenum Press, New York), to be published.
3.59